Hunt — On the Action of Waves on Sea-beaches, 8fc. 257 



the water composing it, even when the water at the highest part 

 of the wave is travelling with its greatest velocity in the same 

 direction as the wave. 



Dr. 8coresbj observed storm-waves in the Atlantic having a 

 period of 1 6 seconds, and a speed of 32-i- miles per hour. For con- 

 venience sake we may consider the hypothetical case of a swell with 

 a period of 15 seconds, a speed of 80 miles, and a height, from 

 trough to crest, of 5 feet. In such a swell the circle described by 

 a surface particle would have, in round numbers, a circumference of 

 15 feet, and the time taken in describing it would be 15 seconds. 

 Thus the speed of the particle in a forward direction on the crest of 

 the wave would be one foot per second, or 3600 feet per hour. But 

 the speed of the wave itself is 30 miles, or 158,400 feet per hour ; 

 from which it follows that the speed of the wave is 44 times the 

 speed of the water composing it, even when the latter, on the crest 

 of the wave, is moving at its greatest velocity in the direction of the 

 wave. Under these circumstances it is clearly impossible for such 

 a wave to draw any of its supplies from the rear. The advancing 

 wave depends entirely for its continued existence on the water in 

 front of it, and on that only. 



From the fact that a particle on the surface of the water moves 

 "in a circle which is described with uniform velocity," it follows 

 that each wave gives rise to two currents that are equal and 

 opposite, the first flowing in a direction contrary to that of the 

 advancing wave, the second flowing in the same direction as the 

 wave. It will be convenient to refer to these currents as the 

 primary and the secondary — terms descriptive of their order of 

 sequence in point of time. 



When a swell such as I have described reaches water sufficiently 

 shallow for it (to use Mr. Russell's expression) " to feel the bottom," 

 tbe primary current, extending as it then does, though with varying 

 intensity, throughout the whole mass of the water from surface to 

 bottom, flows over and in contact with the said bottom, in its pas- 

 sage to meet the advancing crest. The secondary current returning 

 with, and following the crest, affects the bottom in a similar manner, 

 though in an opposite direction. Thus the circular motion of a 

 surface particle in deep water is resolved, so far as the bottom is 

 concerned, into two equal and opposite horizontal currents. The 

 action of these currents on a movable object on the bottom is, in 



SCIEN. PfiOC, R.D.S. — VOL. IV. PT. YI. 2 B 



